How Carbon Negative Cement Works

Before taking a look at the approach Novacem and other environmentally friendly cement companies use, let's examine how regular old Portland cement is made. And by old, we really do mean old: The process for making Portland cement was discovered in the early 1800s by an English stone mason and has essentially remained the same ever since [source: Schenker]. In a nutshell, the 3 billion tons of cement that get transformed into 30 billion tons of concrete each year are produced by first digging up limestone, which is primarily made up of the remains of marine animals [source: Schenker].

Transforming the age-old sea creature shells that make up limestone into cement does two things that lead to substantial carbon emissions. First of all, the limestone must be heated to a temperature of more than 2,500 degrees Fahrenheit (1,371 degrees Celsius) to transform it into cement, a procedure that obviously requires the consumption of a large amount of fossil fuels [source: Prendergast]. At the same time, limestone releases a significant amount of carbon -- calcium carbonate is embedded in limestone -- when it is heated to such high temperatures. In total, the traditional approach to making Portland cement emits almost 1,800 pounds (816 kilograms) of carbon dioxide for each ton of cement made [source: Schenker]. According to many scientists, those increased carbon emissions are in turn prompting a warming of Earth's climate and a host of adverse impacts, such as more severe weather, melting glaciers and rising sea levels.

By contrast, companies creating carbon negative cement use different raw materials and less energy. While the exact process and ingredients are proprietary, this is what Novacem will reveal: Rather than using limestone, the company relies on what are known as magnesium silicates, which come from olivine, talc and serpentine. Importantly, magnesium silicates have no carbon embedded in them, which means there are no emissions when they are heated. There's another benefit in using magnesium silicates instead of limestone as source material: They only need to be heated to about half the temperature of limestone in order to produce cement. A lower temperature, of course, means that less fossil fuels are needed for the process to work -- and that fewer emissions are produced along the way.

There's one more aspect to the process of making Novacem's cement that takes it from just being less harmful emissions-wise to actually being carbon negative. In order to make all the ingredients harden into cement, Novacem adds magnesium carbonates, which are produced by combining carbon dioxide and some other ingredients. Although it's a bit complicated, the lowdown is this: Adding carbon dioxide to the cement that would otherwise be emitted into the atmosphere makes it carbon negative. Indeed, Novacem believes that each ton of cement produced could actually absorb one-tenth of a ton of carbon dioxide [source: Rosenwald].

With backing from the British government and Laing O'Rourke, one of the biggest construction companies in the country, Novacem is probably the highest-profile outfit pursuing carbon negative cement. But, as mentioned on the previous page, it's not the only one. In California, for example, a company called Calera has also received backing from the federal Department of Energy and prominent venture capitalist Vinod Khosla for its efforts. In essence, what Calera does is literally take the carbon dioxide emissions being spewed out of a power plant and mix them with water and other ingredients in order to make carbonates that go into cement. Calera already has a pilot operation in California and, in 2011, announced an arrangement to build a cement factory next to a coal plant in China.

Read on to see what difference carbon negative cement could make -- and what challenges exist for it to become ubiquitous.